The invention concerns a method of compensating the offset voltage in a variable-gain amplifier.
Variable-gain amplifiers are employed for example in focus-and tracking-control loops in the optical scanning systems of compact-disk players. The wanted signals--audio or video signals stored in circular or spiral tracks on the disk for example--employed in compact-disk players are read out with an optical system. The optical system consists as a rule of a system of lenses that are axially displaced by the focus-control loop in such a way that the focus of the lens system rests against the disk. This method of control, which is basically similar to adjusting the image in a telescope, is called focussing. To allow the optical system to read the wanted signals off the disk satisfactorily it is necessary for the focus-control loop to be constantly adjusting the focus. Since even slight deviations in sharpness can lead to errors in reading the wanted signals stored on the disk, the variable-gain amplifier offset voltage cannot be too high, and it must accordingly be compensated.